63篇之24第5题，没找到相关讨论，不知道是否有重合但用了别的编号的讨论，麻烦有知道的指点一下，谢谢

lydiar_020

PASSAGE 24
*All of the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and form structures as different as roots, stems, leaves, and fruits? The answer is that only a
5) small subset of the genes in a particular kind of cell are expressed, or turned on, at a given time. This is accom- plished by a complex system of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified:
10) auxin, abscisic acid, cytokinin, ethylene, and gibberel-lin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharins.
*Unlike the oligosaccharins, the five well-known planthormones are pleiotropic rather than specific, that is,
15)
each has more than one effect on the growth and devel-opment of plants. The five has so many simultaneous effects that they are not very useful in artificially controlling the growth of crops. Auxin, for instance,stimulates the rate of cell elongation, causes shoots to
20) grow up and roots to grow down, and inhibits the growth of lateral shoots. Auxin also causes the plant to develop a vascular system, to form lateral roots, and to produce ethylene.
*The pleiotropy of the five well-studied plant
25) hormones is somewhat analogous to that of certain hormones in animal. For example, hormones from the hypothalamus in the brain stimulate the anterior lobe of the pituitary gland to synthesize and release many different hormones, one of which stimulates the release
30) of hormones from the adrenal cortex. These hormones have specific effects on target organs all over the body.One hormone stimulates the thyroid gland, for example, another the ovarian follicle cells, and so forth.In other words, there is a hierarchy of hormones.
*
35) Such a hierarchy may also exist in plants. Oligosac-charins are fragments of the cell wall released by enzymes: different enzymes release different oligosac- charins. There are indications that pleiotropic plant hormones may actually function by activating the
40) enzymes that release these other, more specific chemical messengers from the cell wall.
5. According to the passage, which of the following bestdescribes a function performed by oligosaccharins?
(A) Regulating the daily functioning of a plant's cells
(B) Interacting with one another to produce differentchemicals
(C) Releasing specific chemical messengers from aplant's cell walls
(D) Producing the hormones that cause plant cells todifferentiate to perform different functions
(E) Influencing the development of a plant's cells bycontrolling the expression of the cells' genes

答案是E，但我即使知道答案也倒推不出来在原文有讲过这个意思，有明白的人帮忙来说说

lydiar_020

ding

cgzjessieli

顶，我也不明白这道题~~谁来讲一下啊？！

miffyflysky

[tr][/tr][td=56][/td]第1段：
[td=325][/td]植物中：同一个细胞变化成不同的功能 全文主旨句
原因：细胞中只有一部分基因被表现出来了
这样的部分表现性有赖于：一个复杂的系统：荷尔蒙和某些分子的存在和新发现分子O

[tr][/tr][td=56][/td]第二段：分子O与五种荷尔蒙的区别
分子如O的单效性
五种荷尔蒙多效性：举例说明

[tr][/tr][td=56][/td]第三段：五种植物荷尔蒙和动物的有相似性
举例说明

[tr][/tr][td=56][/td]第四段：植物荷尔蒙也有这样的层次
荷尔蒙（多效）可刺激不同酶（专效），每种酶释放不同O(专效)
由此说明了O虽然专效，但可由荷尔蒙的多效来实现不同情况下不同表现
（有层次即是说有不同情况下有不同功效，也即是证明第一段的主旨句）

[tr][/tr][td=56][/td][td=325][/td]全文都是在为主旨句服务的。第一段将同一种细胞可表现不同功能的原因递推到荷尔蒙和分子O
[tr][/tr][td=56][/td]　[td=325][/td]因此，要说明主旨句，即要说明荷尔蒙和分子O为什么会在不同情况下表现不同
荷尔蒙：其本身就具有多效性，即不同情况会表现不同
分子O：植物荷尔蒙和动物荷尔蒙相似，存在层次，递推植物中也有层次，这种层次可由引起O的不同表现，即说明了O在不同情况可表现不同

个人愚见。。。

cecilia422

E项确实在文中没有具体体现。。是不是很想选B？这题我就错选B了，后来自己纠结了一会儿想明白了，把思路放在这里，希望指正~

(B) Interacting with one another to produce different chemicals
我一开始觉得小O是受到酶的刺激产生的，那么这就算interact了吧，但是后来看到了一个很厉害的解释

"oligo-saccharin受到enzymes的刺激而产生，算不算interact? 后来想想不算，interact应该是两样东西都存在再interact，而oligo-saccharin和enzymes的产生由先后，且是一个产生了另一个，不是两个interact产生了第三样东西"

而且呀，这里说的是interacting with one another，那也有可能是小O和其他的小O进行interact，可是这点在文中没有提到过

接下来看E:
Influencing the development of a plant’s cells by controlling the expression of the cells’ genes
小O有单一作用，所以能影响到植物细胞的基因表达，从而影响到植物生长。如果排除掉A\C\D之后（A和D在文中没有提过，C很明显是enzyme的作用），只有这个比较贴切了吧~
我觉得这题的难度在于原文没有明确的说小O的作用，只能自己从它的作用比较specific来推